The Pentateuch prescribes only one fast a year, the recurring Day of Atonement; see Lev. 23.27
. The day of the LORD
was the future day of final judgment on the nations, and the purification and restoration of Israel as described in ch. 3
. Some see the plague of locusts as part of that future judgment, which is conceived of as already beginning to arrive.

The LORD suggests the proper response, a genuine repentance (of the heart) and not merely an outward one (garments). For he is gracious: a frequent liturgical formula (compare Pss. 86.15; 103.8; 145.8
). Repent: God frequently has a change of heart; see Gen. 6.6
. Blessing: usually a tangible evidence of divine favor.

The solemn ritual is proclaimed again. The altar stood in front of the temple. Byword: mockery (more generally, a sententious saying, like a proverb). If Israel is not spared, the Gentiles will infer that Israel's
God is ineffectual.